Question: 5 folders cost $5.00. Which equation would help determine the cost of 3 folders?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 3 folders. Since 3 folders cost $x$ , we have the following proportion: $\dfrac{3}{x}$ We can write the fact that 5 folders cost $5.00 as a proportion: $\dfrac{5}{\$5.00}$ The cost changes along with the number of folders purchased, and so the two proportions are equivalent. $\dfrac{3}{x} = \dfrac{5}{\$5.00}$